THE CHEMISTRY OF BACTERIA. 101 



of glucose into lactic acid may be expressed by the 

 equation: 



CeHijOa = 2 CgHeOa. 



This equation is not strictly correct, inasmuch as small 

 amounts of by-products are always present. Sometimes 

 considerable amounts of carbonic acid and of hydrogen 

 may form. 



The lactic acid which usually forms in this kind of 

 fermentation is optically inactive. It does not rotate, 

 either to the right or left, a ray of polarized light. Some 

 organisms, however, give rise to sarco- or para-lactic acid 

 whereas others form levo-rotatory lactic acid. The union 

 of the latter with sarco-lactic acid, which is dextro-rotary, 

 results in the formation of the ordinary inactive lactic acid. 

 It is interesting to note in this connection that under cer- 

 ditions the colon bacillus, which ferments lactose as well 

 as glucose, produces dextro-lactic acid, whereas the typhoid 

 bacillus ferments glucose but not lactose, and gives rise to 

 levo- lactic acid. Depending upon the kind of sugar acted 

 upon, the colon bacillus may give rise to levo-, dextro-, or 

 inactive lactic acid. 



In dental caries, lactic acid fermentation is ascribed an 

 important r61e. The organisms present in the mouth are 

 supposed by Miller, to elaborate lactic acid out of the 

 remnants of starchy food, left between the teeth or at the 

 edge of the gums. This acid then unites with the calcium 

 of the teeth and thus causes decalcification. This first 

 stage is followed by a second, wherein the organic matter 

 of the tooth is softened and dissolved, by the peptonizing 

 bacteria that are present. 



In abnormal fermentations in the stomach (dyspepsia) 

 lactic acid is a common product. These decompositions 

 are apt to occur when the amount of free hydrochloric acid 

 is diminished, or when the food remains too long in 



